A new study estimates that the number of people with dementia is set to double every 20 years as the global population grows and lives longer.

After a systematic review of published studies on Alzheimer's, a panel of 12 international experts has estimated that 24 million people around the world suffer the brain disorder today. They predict this number will rise to 42 million by 2020 and 81 million by 2040.

The estimate was prepared by the Institute of Psychiatry at King's College London and appears in the medical journal Lancet.

In terms of sheer numbers, developing countries are the hardest hit, and the situation will only get worse. The researchers found that 60 percent of dementia patients live in poorer nations, with the proportion set to rise to more than 70 percent by 2040.

To put it another way, the number of Alzheimer's patients is expected to increase 300 percent in China, India, and their southeast Asian and western Pacific neighbors during this period compared to 100 percent for the industrial states.

"When we think about developing countries, we think of children," said he report's lead author, psychiatrist Cleusa Ferri But these populations are aging. "It's not just that the population is very high, but the proportion of old people is increasing. People are living longer in these countries."

Although absolute numbers of dementia cases are expected to grow faster in developing nations, their prevalence rates, the proportion of the elderly suffering it, is still lower than in the rich countries. The authors speculate that this could be because of lower survival rates rather than lower incidence.

But they say it could also be because of under-reporting. Dr. Ferri says dementia is a hidden problem in developing nations.

"People think it is not a disease," he said. "They think it is aging, so they don't seek help, or even if they seek help, most of the countries don't offer services that meet their needs."

Dr. Ferri and her colleagues say they hope their projections will help policymakers plan health services in the future. They argue that governments should allocate resources to prevent behaviors and conditions that increase the chances for heart disease, a risk factor for Alzheimer's. These include smoking, high blood pressure, high blood cholesterol levels, and diabetes.